Current air pollution regulations in most industrialized parts of the world are very restrictive concerning the amounts of H.sub.2 S industry can discharge into the atmosphere. Los Angeles, for example, requires that no more than 10 ppmv be so discharged. The discharge requirements of SO.sub.2, however, are not nearly so restrictive; Los Angeles waste gas streams containing up to 500 ppmv SO.sub.2 can be safely discharged while Canada and Germany allow up to 2000 and 4000 ppmv, respectively. As a result, there is provided by law a strong incentive for industries engaged in such diverse activities as petroleum refining, meat packing, soap production, sewage treatment, electrical generation and chemical production to convert the H.sub.2 S in their waste gas streams to SO.sub.2 prior to atmospheric discharge. The manner by which this is presently accomplished is through the use of a stack gas incinerator, i.e., by blending sufficient natural gas or other fuel with the waste gas stream to provide a combustible mixture, and then burning the resultant mixture in the temperature range of 1350.degree. - 1550.degree. F at the point of discharge.
With the advent of the energy crisis another incentive has been provided to industry - namely, that of saving expensive fuel. Ideally, it would be most desirable to oxidize the H.sub.2 S to SO.sub.2 catalytically, without adding fuel. The development of a catalytic incinerator, however, has been hampered by the fact that waste gas streams containing H.sub.2 S usually also contain in uncombustible amounts such highly oxidizable components as CO, H.sub.2 and light hydrocarbons. These components, however, as well as H.sub.2 S, are readily oxidized by catalysis and in the process release large quantities of heat, thus requiring the catalytic incinerator to employ cooling means to avoid detrimental temperature increases. If the temperature should exceed 900.degree. F, H.sub.2 S may be produced (as will be explained hereinafter); if it exceeds about 1000.degree. F there are the further dangers that SO.sub.3 may be produced and that the catalyst might be thermally destroyed. Consequently, most catalytic incinerators need cooling facilities which may necessitate as much energy input, and more maintenance requirements, than burning in a stack gas incinerator.
From the preceding discussion, it is apparent that for a catalytic incinerator to be most effective it must be selective for the oxidation of H.sub.2 S to SO.sub.2. A major objective of the invention, therefore, is to provide a novel process for selectively incinerating H.sub.2 S in the presence of other normally oxidizable components. Another objective is to provide novel catalysts for effecting the selective incineration of H.sub.2 S to SO.sub.2. Another objective is to utilize the catalytic incineration process of the invention for treating vent gases emanating from geothermal power plants.
The present invention is a revised version of a known catalytic incineration process largely abandoned by the art. In United Kingdom Pat. No. 733,004, published Jan. 23, 1953, it is taught that a catalyst composed of 5 - 10 weight-percent V.sub.2 O.sub.5 on alumina is effective in reducing H.sub.2 S concentrations in Claus tail gas streams by converting the same to SO.sub.2 ; however, no mention is made therein that said catalyst is selective for the incineration of H.sub.2 S to SO.sub.2 in the presence of H.sub.2, CO or light hydrocarbons.
By the process of the present invention it has been found that catalysts composed of 5 - 15 weight-percent V.sub.2 O.sub.5 on alumina, hydrogen mordenite or any other non-alkaline porous refractory oxide are very selective for the oxidation of H.sub.2 S to SO.sub.2 in the presence of H.sub.2, CO or light hydrocarbons. Even more surprisingly, it has been found that these normally oxidizable components of H.sub.2, CO and light hydrocarbons remain unoxidized even when excess air is utilized to perform the H.sub.2 S to SO.sub.2 conversion. Furthermore, conversions of H.sub.2 S to SO.sub.2 are essentially 90 to 100 percent complete and space velocities varying in the wide range of 1,000 to 100,000 GHSV can be utilized. Operating temperatures can vary from a minimum of 300.degree. to a maximum of about 900.degree. F.